1. Field of the Invention
This invention relates to mobile telecommunications systems, and more particularly, to a method and system for building a neighbor cell measurement channel list for mobile station handoff.
2. Description of the Prior Art
In a cellular mobile telecommunications system the user of a mobile station communicates with the system through a radio interface while moving about the geographic coverage area of the system. The radio interface between the mobile station and system is implemented by providing base stations dispersed throughout the coverage area of the system, each capable of radio communication with the mobile stations operating within the system. In a typical mobile telecommunications system each base station of the system controls communications within a certain geographic coverage area ideally represented by a hexagonal shape termed a cell, and a mobile station which is located within a particular cell communicates with the base station controlling that cell. When a call is initiated by the user of a mobile station, or received at the system for a mobile station, the call is set up on radio channels assigned to the base station controlling the cell in which the mobile station is located. If the mobile station moves away from the original cell in which the call was setup and the signal strength on the radio channels of the original cell weakens, the system will affect transfer of the call to radio channels of a base station controlling a neighboring cell into which the mobile station moves. As the mobile station user continues to move throughout the system, control of the call may be transferred from the neighboring cell to another cell. This transfer of the call from cell to cell is termed handover or handoff.
Handoff can only be effective if the call is transferred to radio channels that provide adequate signal strength for two way communications. This requires sufficient signal strength at both the receiver of the mobile station and receiver of the base station to which handoff is made. The signals must also be sufficiently strong in relation to any noise or interference that is present in the network. For effective handoff it is necessary that some sort of signal strength or interference level measurement process be used to determine which of the neighboring cells is to be selected for handoff. In existing systems the measurement process is done by either making measurements at the receivers of neighboring base stations on signals transmitted from the mobile station, or by making measurements at the receiver of the mobile station on signals transmitted from neighboring base stations. The latter method requires that the mobile station be a part of the measurement process used to select a cell for handoff.
In analog cellular systems operating according to the EIA/TIA--553 Mobile Station--Land Station Compatibility Specification (AMPS) handoff measurements are done solely at neighboring base stations. When signal measurements made at the base station providing coverage in the current cell indicate that the strength of the signal received from a mobile station involved in a call has fallen below a certain threshold, the base station informs the mobile switching center (MSC) in control of the system or system area in which the cell is located. The MSC then initiates the handoff measurement process. The MSC orders base stations of neighboring cells to monitor the signal transmitted by the mobile station on the current radio channel assigned to the call, and measure the strength of the signal. After receiving the measurement results from each base station that received the measurement order, the MSC will then initiate handoff of the call from the current cell to the cell containing the base station reporting the highest received signal strength. The MSC uses a "neighbor cell list" that is associated with the current cell to determine which base stations receive the measurement order from the MSC. The neighbor cell list is created and set manually by the system operator and remains fixed until the operator later manually modifies the list. This type of handoff measurement process requires a large amount of signaling traffic between the MSC and the base stations of the cells contained in the neighbor cell list. This signaling traffic further consumes many processing and signaling link resources. For this reason the number of cells included in the neighbor cell list must be limited. The list is typically configured to include the cells which immediately border the current cell. If hexagonal cell shapes of identical size are used to model the system cells' coverage area there will be six bordering cells.
The nature of the operation of digital cellular communications systems, as opposed to analog cellular systems, allows that the handoff measurement process be performed at the mobile station. An example of these types of systems, are systems operating according to the IS-54B EIA/TIA Cellular System Dual-Mode Mobile Station (IS-54B)--Base Station Compatibility Standard. In IS-54B systems the time division multiple access (TDMA) signal transmission mode is used. In TDMA, communications between a base station and a particular mobile station are transmitted on radio channels that also may be used for communications between the same base station and up to two different mobile stations. The communications are carried out through data or digitized voice signals that are transmitted as bursts in timeslots that are time multiplexed on the radio channels. Each mobile station in communication with a base station is assigned a timeslot on both the reverse channel and forward channels. The assigned timeslots are unique to each mobile station so communications between different mobiles do not interfere with each other. The handoff process in IS-54B is known as mobile assisted handoff (MAHO). In MAHO handoff measurement is done at the mobile station during the times when the mobile station is neither transmitting in the assigned reverse channel timeslot nor receiving in the assigned forward channel timeslot. During the times between signal bursts in an ongoing call, the mobile station periodically monitors radio channels of each base station located in close proximity. The control channel of each neighboring base station is typically used as the measurement channel. For each ongoing call the measurement channels are contained in the neighbor cell list of the cell in which the call is proceeding. In addition to measuring the measurement channels of neighboring base stations, the mobile station also measures the received signal strength on the current channel on which the call is proceeding. The mobile station measures the received signal strength on these radio channels and transmits the measurement results to the current base station. The current base station then forwards these measurement results to the MSC. If the received signal strength on the current channel falls below the received signal strength on a measurement channel of a neighboring cell the MSC initiates handoff to that neighboring cell.
The analog control channels (ACCH) of the neighboring cells are used as the measurement channels for IS-54B MAHO. With the introduction of the new IS-136 EIA/TIA Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard (IS-136), which is essentially the IS-54B standard with a digital control channel introduced, it is also possible to use a digital control channel (DCCH) for MAHO measurements.
Because MAHO is performed mostly within the mobile station the resources for carrying out the process are limited. IS-54B or IS-136 mobile stations can perform only fifty measurements per second. Radio conditions such as Rayleigh fading, shadowing, etc. are such that it is necessary to average measurements in order to provide a reliable signal strength value. Therefore it is necessary to limit the number of cells that comprise the neighbor cell list for MAHO measurement purposes to much less than fifty cells. The IS-54B standard limits the size of the neighbor cell list to twelve cells. IS-136 sets a size limit of twenty four. The increase in the size of the list in IS-136 over IS-54B has limited effect because the limit of fifty measurements per second still holds and any increase in the number of cells in the list dilutes signal strength measurement precision on any given measurement channel.
When a system operator creates a neighbor cell list for a cell he attempts to ensure that calls in the cell can be handed over to a second cell, no matter what type of movement takes place. One of the difficulties in creating a neighbor cell list is that the actual coverage area of a cell is difficult to predict. The size and shape of a cell's coverage area may vary due to various effects. Examples of such effects are, base station antennas being located in different positions, or shadowing effects on radio coverage caused by obstacles such as buildings. Although the ideal representation of the coverage area of a cell may be a hexagonal cell having six neighbors of identical shape, the actual coverage areas of cells have differing sizes and shapes. The best candidate cell for handoff may not always be one of the six bordering cells as depicted in the ideal representation when cells within a system are modeled as being of equal size. It is possible that the best candidate for handoff would be a cell located beyond the six bordering cells. In the ideal representation this could be one of the twelve cells adjacent to the ring formed by the six bordering cells. Since it is difficult to predict the actual coverage area of each individual cell in a system, it would be very difficult to create a neighbor cell list for the handoff process in the above situation. Without knowing the actual coverage area of the base stations it would be necessary to include all eighteen of these cells in the neighbor cell list to create the most accurate list for handoff measurements. In EIA/TIA--553 and IS-54B systems it is not possible to include all of these eighteen cells in the neighbor cell list. In IS-136 systems, although the standard allows eighteen cells in the neighbor cell list, the number is excessive and the precision of signal strength measurements would not be as great as it could be.
It would provide an advantage then in a cellular system to have a method and system for creating a neighbor cell list that accounts for the differing coverage areas of cells. A method and system of this type would allow creation of a neighbor cell list that would contain the best possible candidate cells for handoff of a call. The method and system would also allow creation of a neighbor cell list of a size that allowed accurate handoff signal strength measurements, while still providing the best possible candidate cells for a list of that size. Automation of the method and system would free the system operator from having to manually create a neighbor cell list for a new cell or, from having to manually reconfigure the neighbor cell list of affected cells when a new cell is added to a system. The present invention provides such a method and system.